1. Field of the Invention
This invention relates to a method of making an inverted-hook groove for a plant-fiber molded cup lid, particularly to one of forming an inverted-hook groove around the side edge of the plant-fiber molded cup lid that is thermally compressed and shaped. Thus, the plant-fiber molded cup lid formed with an inverted-hook groove can be tightly and closely combined with the upper edge of a primary cup body.
2. Description of the Prior Art
A so-called “plant-fiber molded cup lid” is made of plant fiber, which is mostly made of paper pulp, but other composite materials of plant fiber are also available. A cup lid formed by compression molding is commonly called as a “paper molded cup lid.” At present, in plant fiber (paper pulp) molding trade, shaping (molding) of a product is carried out in two ways of thermal pressure. The first way is to have an unfinished paper-pulp molded cup lid thermally compressed by means of thermal compressing-shaping equipment when the water content of the molded cup lid is about 55-70%. The temperature of thermal compression is 150° C.±10° C. and the time of thermal compression depends on the volume, that is, the larger the volume is, the longer the time of thermal compression must be. Generally, the time of thermal compression for a paper-pulp molded cup lid is 45 s±5 s. The second way is to have an unfinished paper-pulp molded cup lid dried by heat when the water content of the cup lid is about 55-70%, and then let the cup lid thermally compressed and shaped when its water content rate drops to 7-10%. The temperature of thermal compression is 150° C.±10° C., which is the same as that of the first way, but the time of thermal compression is shorter than that of the first way. Substantially, thermal compressing and shaping is a technique customarily employed for carrying out molding of plant fiber (paper pulp). For carrying out thermal compression molding work, it is necessary to employ an upper mold and a lower mold that are tightly closed together, and the upper and the lower mold have to be moved for a distance when they are closed and opened; therefore, in order to release a shaped product from the mold smoothly, the interior of the shaped product cannot produce a structure of negative angle. As a result, a paper-pulp molded cup lid cannot produce an inverted-hook groove to be tightly and closely combined with the upper edge of a cup body.